Efficient Diodes Research Articles

Light-Emitting Diodes Based on Two-Dimensional Nanoplatelets

Colloidal semiconductor nanocrystals (NCs) attract significant interest in recent years due to their narrow and tunable emission wavelength in the visible range, as well as high photoluminescence quantum yield (PLQY), which are highly desired in display technologies. The high-quality NCs have been recognized as vital luminescent materials in realizing next-generation display devices. With further development, NCs with near-unity PLQY have been successfully synthesized through engineering of the core/shell heterostructure. However, as the external quantum efficiency (EQE) of the nanocrystal light-emitting diodes (LEDs) approaches the theoretical limit of about 20%, the low out-coupling factor proposes a challenge of enhancing the performance of a device when using the spherical QDs. Hence, the anisotropic NCs like nanoplatelets (NPLs) are proposed as promising solutions to improve the performance of nanocrystal LEDs. In this review, we will summarize the synthetic strategies of two-dimensional (2D) NPLs at first. Then, we will introduce fundamental concepts of LEDs, the main approaches to realize LEDs based on nanoplatelets, and the recent progress. Finally, the challenges and opportunities of LEDs based on anisotropic NCs are also presented.

Improve the Efficiency of Organic Light-Emitting Diodes Using a Novel Hole Injection Layer with Triple-Layer Gradually Doped Structure

Improve the Efficiency of Organic Light-Emitting Diodes Using a Novel Hole Injection Layer with Triple-Layer Gradually Doped Structure

Enhanced optical output of GaN-based near-ultraviolet light-emitting diodes using an ultra-thin air cavity nanopatterned sapphire substrate

A combined methodology is proposed to fabricate nanopatterned sapphire substrate with air cavity, which is used to improve the crystalline quality and light extraction efficiency of near-ultraviolet light-emitting diodes.

Breakthrough in Efficiency of Exciplex Organic Light-Emitting Diodes Using 9-Phenylfluorene Based Electron Acceptors

Breakthrough in Efficiency of Exciplex Organic Light-Emitting Diodes Using 9-Phenylfluorene Based Electron Acceptors

Luminance and efficiency of a light-emitting diode with transport-blocking poly(methyl methacrylate) layers and quantum dots: theoretical model, experiment, optimization

The results of studies of the influence of the thicknesses of the electron-blocking, hole-conducting and recombination layers on the luminance-voltage and other electrophysical characteristics of a multilayer light-emitting diode with an active layer based on semiconductor quantum dots are presented. Optimized thicknesses and voltages are determined to achieve the maximum efficiency of electroluminescence of the light-emitting diode. Keywords: quantum dots, light-emitting diode, blocking layer.

Rational design of blocking groups for high triplet energy n-type host materials

The effect of blocking groups on the device lifetime and efficiency of blue phosphorescent organic light-emitting diodes (PhOLEDs) was investigated by synthesizing electron transport-type hosts with different blocking groups.

A zero bias highly efficient active diode circuit for piezoelectric energy harvester

This paper presents a MOS based active diode circuit for efficient rectification of piezoelectric energy harvester (PZEH). The active diode consists of a PMOS switch along with auto tuned control circuit to control the gate of the PMOS. The strength of gate voltage of the PMOS in conducting state has been improved to provide the low forward resistance at high current. In the on-state, the voltage drop across the active diode is 33 mV at diode current of 136 µA. The maximum reverse leakage current in the non conduction state of PMOS has been reduced from 18 µA of conventional MOS diode to 1.5 µA. The auto tuned control circuit does not require external bias. The active rectifier using proposed active diode used for ac to dc conversion of PZEH output has been presented in this paper. The circuit provides a power conversion efficiency of 94% which is 36% better than the conventional MOS based rectifier.

A zero bias highly efficient active diode circuit for piezoelectric energy harvester

A zero bias highly efficient active diode circuit for piezoelectric energy harvester

Enhancing the Luminance Efficiency of Formamidinium-Based Dion-Jacobson Perovskite Light-Emitting Diodes via Compositional Engineering.

In this paper, the phase formation mechanism of formamidinium (FA)-based Dion-Jacobson (DJ) perovskites is uncovered for the first time, which includes the formation of the n1 domain (n = 1 perovskite phase) and a trace amount of the large n domain in the spin-coating process and the growth of the n2 domain and the large n domain during the thermal annealing stage. The phase formation mechanism clearly reveals the different phase distributions between FA-based Ruddlesden-Popper (RP) and FA-based DJ perovskite films at different stages of film preparation due to the larger formation energy of the DJ perovskites. According to this phase formation mechanism, we put forward an effective strategy of the small A-site cation compositional engineering. Then, excess perovskitizer cations (FA+) are introduced to increase the crystallinity, modulate the phase distribution and passivate defects without disturbing the structure of DJ perovskites simultaneously. The final green-light DJ PeLED devices show a maximum luminance of 41 520 cd/m2 and a maximum current efficiency of 31.1 cd/A (EQE: 8.5%), which are the record values so far. The final DJ PeLEDs show an improved operational lifetime of ca. 15 min at an initial luminance of ca. 800 cd/m2. Our results suggest that DJ perovskites can be promising for light-emitting applications.

Chiral Conjugated Thermally Activated Delayed Fluorescent Polymers for Highly Efficient Circularly Polarized Polymer Light-Emitting Diodes.

Two novel chiral conjugated polymers R-P and S-P designed and synthesized from a pair of circularly polarized thermally activated delayed fluorescence (CP-TADF) enantiomers are presented in this work. The two polymers exhibited excellent TADF properties with small singlet-triplet energy gaps (ΔEST) of 0.045 and 0.061 eV and relatively high photoluminescence quantum yields (PLQYs) of 72 and 76%, respectively. Besides, intense mirror-image circularly polarized luminescence signals were detected from R-P and S-P in both solution and film states with dissymmetry factors (|glum|) of up to 1.9 × 10-3. Furthermore, solution-processed circularly polarized polymer light-emitting diodes (CP-PLEDs) fabricated with R-P and S-P achieved high maximum external quantum efficiencies of 14.9 and 15.8% and high maximum brightness (Lmax) of 8940 and 12,180 cd/m2 with yellowish-green emission peaks at 546 and 544 nm, respectively. Moreover, intense circularly polarized electroluminescence signals with electroluminescence dissymmetry factors (gEL) of -1.5 × 10-3 and +1.6 × 10-3 were detected from the CP-PLED devices fabricated with R-P and S-P, respectively.

Microscale gratings patterned in perovskite films to promote the electron-optical conversion efficiency of perovskite light-emitting diodes.

In this paper, we investigated an enhancement of luminance in a perovskite light-emitting diode (PeLED) by inserting periodic microscale gratings. Soft imprinting was employed to pattern the gratings onto the surface of perovskite film in the annealing process. It was found that the grain-refining effect deeply influenced the morphology and crystallinity of the perovskite film. The finite-element analysis was utilized to confirm the light extraction enhancement arising from the inserted microscale gratings. Additionally, the introduction of microscale gratings led to a 1.33 enhancement of the device's optoelectrical field intensity. Thus, the patterned PeLED obtained an enhancement of 2.96 and 2.1 of the luminance and external quantum efficiency, respectively.

Virtual Screening of TADF Emitters for Single-Layer OLEDs.

Thermally-activated delayed fluorescence (TADF) is a concept which helps to harvest triplet excitations, boosting the efficiency of an organic light-emitting diode. TADF can be observed in molecules with spatially separated donor and acceptor groups with a reduced triplet-singlet energy level splitting. TADF materials with balanced electron and hole transport are attractive for realizing efficient single-layer organic light emitting diodes, greatly simplifying their manufacturing and improving their stability. Our goal here is to computationally screen such materials and provide a comprehensive database of compounds with a range of emission wavelengths, ionization energies, and electron affinities.

Optimizing the efficiency of a laser diode and single-mode fiber coupling using multi-aspherical lenses

The continuous increase in the network access rate requires a higher integration of optoelectronic devices for optical communication. Optical devices require a longer optical path and an enhanced power budget. Therefore, this study designs a three-lens coupled optical path for integrating multiple laser diodes and photodiodes with a high coupling efficiency. The coupling efficiency between the laser diode, aspherical lenses, and single-mode fibre (SMF) is analysed using the physical ray-tracing method. In the simulation results, the maximum coupling efficiency of the system reaches 72.8% by optimising the parameters of the optical path. In the experimental results, the maximum coupling efficiency is 68.01%, indicating the effectiveness of the theoretical approach and proposed design scheme. Furthermore, the simulated results indicate that a higher coupling efficiency (79.41%) would be achieved by increasing the core diameter of the SMF under the same conditions. Additionally, using the same divergence angle of the laser diode (79.04%) or decreasing the focal length of the lens (77.24%) can achieve a higher coupling efficiency. These results provide an important reference point for the design and manufacture of laser chips and SMF.

Analyses of the light extraction efficiency of organic light-emitting diodes with ITO/mesh-Ag/ITO anode and its optical transmittance as transparent electrode

We have used the three-dimensional finite-difference time-domain simulation method to analyze the out-coupling efficiency of organic light-emitting diodes (OLEDs) with an ITO/Mesh-Ag/ITO anode. The period and grid space of the meshed Ag structure effectively affected the extraction ratio (ER) of the bottom-emitting OLED. The 3D simulation results for a simple structure of OLED with an ITO/Mesh-Ag/ITO anode showed the ER of about 26.4% and it enhanced 2.26 times that of the continuous Ag layer of about 11.7%. In addition, in respect to the transparent electrode of the OLED using the ITO/Mesh-Ag/ITO multiple layers, the Ag thickness and open ratio were important factors that affected the transmittance and the electrical conductance. The open ratio of 60% resulted in sheet resistance of less than 25 Ω/□, while keeping the transmittance nearly similar to that in the case of a single ITO layer.

Boosting Efficiency of InP Quantum Dots-Based Light-Emitting Diodes by an In-Doped ZnO Electron Transport Layer

InP quantum dots (QDs)-based light-emitting diodes (QLEDs) are considered as one of the most promising candidates for environmentally cadmium (Cd)-free electroluminescence devices. However, the performance of InP QLEDs still lags far behind that of Cd-containing QLEDs, which limits their practical applications in next-generation displays and lighting. Here, we report an all-solution processed green InP QLED, which is enabled by an electron transport layer (ETL) of In-doped ZnO (IZO) nanoparticles (NPs). The ETL of IZO NPs can not only suppress the exciton quenching of InP QDs emitting layer due to the reduced defect states, but also improve the charge balance by partially blocking the injection of electrons, and thus the device performance. The optimized InP QLED exhibits a maximum external quantum efficiency (EQE) of 5.42% corresponding to a current efficiency (CE) of 21.22 cd A <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , which is three times higher than that of the control device based on ZnO ETL, respectively. Our work suggests that IZO NPs can function as a good ETL material in QLEDs and other optoelectronic devices.

Coupling efficiency of a laser diode to a single-mode fiber via a microlens on the fiber tip

A conical microlens on the tip of a fiber for improving the coupling efficiency (CE) of a laser diode and a single-mode fiber in butterfly packaging was experimentally fabricated via the grinding and polishing method and theoretically designed using the beam propagation method. The CE, varying with working distance, was explored under all kinds of optical coupling system parameters, such as incident wavelength, laser beam waist radius, numerical aperture of the fiber, refractive index, and diameter of the microlens. The optimal working distance for the maximal CE was located at approximately 15 μm, proving that the conical-tip coupling was a tight packaging type. Therefore, the CE trends under misalignment and tolerance adjustments could be explained by the spherical aberration theory and light-gathering ability of the microlens. The results are useful in optimum launch optics for designing and fabricating microlens tips on coupled packaging of optoelectronic devices.

Halogen free solvent processed light-emitting diodes achieving EQE nearly 25% for imidazole-based host materials synthesized by ball milling

Although lab-scale organic light-emitting diode (OLED) efficiencies continue to increase, most systems use halogenated solvents to dissolve the emitting layer. Halogenated solvents achieve better film morphologies and efficiencies due to their better solubility for organic semiconductors, but they often have negative environmental impacts. Therefore, we have designed and synthesized three highly efficient new host materials for solution processable green phosphorescent OLEDs (PHOLEDs) using non-halogenated solvents. We utilized environmentally friendly high energy ball milling, with strong mechanical forces producing desired host materials with high reactivity. Impaction and friction followed by mechanical forces also heat the reactants during synthesis. All three materials exhibited excellent photoluminescence quantum yields and high triplet energy for efficient energy transfer to guest molecules. Optimized PHOLEDs by solution processing using halogenated solvent exhibited highest external quantum efficiency (EQE) and current efficiency (CE) of 25.20% and 96.85 cd A−1 with highest illumination of 65931 cd m−2, correspondingly. PHOLEDs utilizing non-halogenated solvents also achieved high EQE and CE (24.70% and 93.07 cd A−1, respectively), currently the highest reported amongst solution processed PHOLEDs utilizing both halogenated and non-halogenated solvents.

A 91.15% Efficient 2.3–5-V Input 10–35-V Output Hybrid Boost Converter for LED-Driver Applications

This article presents a hybrid boost converter architecture for improving the efficiency of light-emitting diode (LED) drivers used in mobile applications. By cascading a low-switching frequency time-interleaved series–parallel switched-capacitor (SC)-stage with an inductive-boost converter, we facilitate lower voltage-rated switches, thus significantly reducing the switching losses. Charge-sharing losses of the SC stage are minimized by soft-charging flying capacitors with the inductor of the boost (BST) stage. Fabricated in 180-nm bipolar CMOS DMOS (BCD) process, the prototype converter generates 30-V output voltage from a Li-ion battery source. It can provide a load current in the range of 0–100 mA with an excellent peak power efficiency of 91.15% at 30 mA, which represents a 3% improvement over the state of the art.

Thermal diode based on a multilayer structure of phase change materials

Thermal diodes are devices that allow heat to flow preferentially in one direction. This unique thermal management capability has attracted attention in various applications, like electronics, sensors, energy conversion or space applications, among others. Despite their interest, the development of efficient thermal diodes remains still a challenge. In this paper, we report a scalable and adjustable thermal diode based on a multilayer structure that consists of a combination of phase change and phase invariant materials. We applied a parametric sweep in order to find the optimum conditions to maximize the thermal rectification ratio. Our simulations predicted a maximum thermal rectification ratio of ~20%. To evaluate the impact of these devices in real applications, we theoretically analysed the performance of a magnetocaloric refrigerating device that integrates this thermal diode. The results showed a 0.18 K temperature span between the heat source and the heat sink at an operating frequency of 25 Hz.

Suppressing exciton deconfinement and dissociation for efficient thermally activated delayed fluorescence OLEDs

The efficiency of organic light-emitting diodes that utilize the principle of thermally activated delayed fluorescence (TADF) depends sensitively on the host material in which the TADF emitter molecules (guests) are embedded. Potential loss processes are “deconfinement,” the transfer of excitons from the guest to the host, and “dissociation,” the formation of intermolecular charge-transfer states. We investigate how both processes can be suppressed by studying the photoluminescence efficiency, emission spectrum, and time-resolved emission intensity of eight thin-film systems in which 5 mol. % of the sky-blue TADF emitter 4-carbazolyl-methylphthalimide (abbreviated here as CzPIMe) is embedded in various host materials. Deconfinement is found to be entirely suppressed if the triplet energy of the host is 0.25 eV or more above that of the guest. For systems allowing for deconfinement, the dependence on the energy difference is consistent with a recent theoretical analysis [C. Hauenstein et al., J. Appl. Phys. 128, 075501 (2020)]. Dissociation, due to hole transfer to a host molecule, is found to be suppressed if the host’s highest occupied molecular orbital energy is not more than about 0.2 eV higher than that of the guest. Otherwise, we observe an efficiency loss, a spectral redshift, and the disappearance of distinct prompt and delayed emission regimes. A comprehensive rate-equation model is developed from which we study the sensitivity of these observations to the energy level structure, the intermolecular interaction rates, and the photophysical rates that follow from a fit to the experimental data for the CzPIMe:TCTA[tris(4-carbazoyl-9-ylphenyl)amine] system.